Jan. 19.1924 
Anatomy of the Sweet Potato Root 
163 
parts throughout the vertical extent of the organ. (PI. 3, B.) Certain 
varieties possess characters of all of these groups and can not be readily 
classified. 
However anomalous the grosser morphological structure of Ipomoea 
batatas may be, the tissues and cell forms are similar to those of other 
dicots. A superficial periderm replaces in protective function the root 
epidermis. (PI. 2, D.) The thin-walled cells composing it form parallel¬ 
epipeds with the shortest diameter in the radial axis of the organ. The 
outer periderm cells become partly lignified and are progressively sloughed 
off. The periderm increases in thickness by a phellogen which remains 
active throughout the season, forming new cells to replace those which 
are sloughed off as the surface of the fleshy root expands. 
The cortex of the mature root is made up almost altogether of secondary 
tissue. The cells composing it are large starch-filled parenchyma cells, 
among which at intervals radial 
rows of sieve-tubes with their 
companion cells are interspersed. 
The sieve-tubes are commonly 
large. The length and course of 
the cells, however, show great 
variations in the regions of 
anomalous growth; it is difficult 
to find a straight row of any ap¬ 
preciable length. The end walls 
of the sieve-tubes are normally 
strictly transverse, but in regions 
of anomalous growth the end 
walls may be sharply inclined 
and even two to three large lateral 
sieve-plates may be developed. 
The companion cells are promi¬ 
nent, of small diameter, but in 
length often equal to their sister 
cells, the sieve-tubes. Adjacent 
to the sieve-tubes and, like them, 
a product of the cambium, are 
latex tubes. (Fig. 5.) These 
vessels occur in single vertical 
Fig. 5.—Cross section of a radial row of secondary sieve- 
tubes with their companion cells. Notice the large 
latex vessel to the left of the row of sieve-tubes. 
rows. The individual segments retain the division walls; in other words, 
resorption of the cross walls which is observed in other plants does not 
take place. In young tissue the latex vessels are very prominent and 
stand out from the surrounding tissue by their larger size and square 
shape in cross section. The cells have a large nucleus and a white, 
viscid fluid in which oil drops are suspended. In older tissues the 
latex tubes are less prominent, since the cells of the tissue surrounding 
them are of equal size or larger. 
The cells of the cambium are typical tracheid-like elements and closely 
resemble those of the common dicots. (PI. 2, A, B, C, and fig. 6.) In 
regions of extreme anomalous growth, however, the cells assume various 
configurations. 
All secondary xylem is composed of wide and narrow vessels. These 
elements have the typical pitted wall structure and communicate with 
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